1. Field of the Invention
This invention pertains generally to the field of aircraft traffic collision avoidance systems and/or position reporting systems.
2. Description of the Related Art
The ability to fly military aircraft in a close formation provides a tactical advantage to the pilots. Some aircraft, such as a helicopter, fly Nap-of-the-Earth which may be considered a type of very low-altitude course used by military aircraft to avoid enemy detection and attack in a high-threat environment. During NOE flight, geographical features are used as cover, exploiting valleys and folds in the terrain by flying in them rather than over them. Although preferable for many missions, it may not be possible for pilots to fly in a close formation where flight operations encounter “zero-visibility” conditions where pilots may lose sight of the other aircraft—even in close formation.
Link 16 is a military tactical data exchange network. With Link 16, military aircraft, ships, and ground forces may exchange their tactical picture in near-real time. Also, Link 16 supports the exchange of text messages, imagery data, and provides two channels of digital voice (2.4 kilobits per second (“kbps”) and/or 16 kbps in any combination). Link 16 operates in the radio navigation frequency band between 960 MHz and 1215 MHz, inclusive, a band of frequencies within the upper UHF frequency range. In Link 16, information is typically passed at one of three data or symbol rates: 31.6 kbps, 57.6 kbps, or 115.2 kbps, although the radios and waveforms can support throughputs of 238 kbps.
Link 16 information is primarily coded in a “J-series messages” protocol comprised of binary data words. These data words are grouped in functional areas and allocated to network participating groups, i.e., virtual networks, such as, but not limited to, Precise Participant Location and Identification (“PPLI”) (categorized as network participation groups 5 and 6), Surveillance (group 7), Command or Mission Management/Weapons Coordination (group 8), Aircraft Control (group 9) and Electronic Warfare and Coordination (group 10).
Multifunction Advanced Data Link (“MFDL”) is comprised of remote electronics. MFDL systems include phased Array Antenna Assemblies (“AAA”) and three Antenna Interface Units (“AIUs”), and allow aircraft to communicate within and between flight groups in order to share a common view of the battle space.
A Traffic Collision Avoidance System (“TCAS”) is an aircraft system designed to reduce the incidence of mid-air collisions. The system monitors the airspace surrounding the aircraft for other aircraft equipped with an active transponder that is independent of air traffic control. With such system, pilots may be alerted of the presence of other transponder-equipped aircraft. The system may be comprised of a TCAS computer unit, antennas, and a display unit. The TCAS computer unit performs, in part, airspace surveillance. The antennas may include a directional antenna mounted to the top of the aircraft and either an omnidirectional or a directional antenna mounted to the bottom of the aircraft. Two additional antennas are required for a Mode S transponder, where one antenna is mounted to the top of the aircraft and the other is mounted to the bottom. These Mode S antennas enable the Mode S transponder to receive interrogations at the radio frequency of 1030 MHz and reply to the received interrogations at 1090 MHz.
An Automatic Dependent Surveillance-Broadcast (“ADS-B”) system is a surveillance system for tracking aircraft. The ADS-B may be comprised of three components: a transmitting subsystem which includes message generation and transmission functions at ownship, a transport protocol comprised of a VHF datalink (“VDL”), a 1090 MHz Mode S Extended Squitter (“ES”), a Universal Access Transceiver (“UAT”), and a receiving subsystem which includes message reception and report assembly functions at the receiving destination such as target aircraft.
The VDL provides a means of sending information between aircraft and ground stations. In VDL Mode 4, aircraft use a band of frequencies between 117.975 MHz and 137 MHz in the VHF frequency range to send messages between large numbers of aircraft as well as a ground station. VDL Mode 4 employs a Self-Organized Time-Division Multiple Access (“STDMA”) protocol that allows a means for self-organizing or managing a network of aircraft employing the STDMA protocol. The STDMA protocol divides channel time into fixed-length time slots. An important term in VOL Mode 4 channel management is “superframe” which consists of a group of slots that span a period of 60 seconds, an extremely high latency rate. The superframe contains a default number of 4,500 slots, an equivalent of 75 slots per second.
An important feature of the protocol is the method used to select slots of a new transmission for placing reservation for future transmissions. The method includes “slot idle/busy notifications” where a station is considered “idle” if an idle/busy status is idle at the start of the slot, “busy” if the status is busy, or “occupied” if the channel is continuously busy for a period of at least 5 milliseconds during the slot. When a channel is not busy, slot selection is straightforward since a slot that has not been previously reserved by another station can be easily found. When a channel becomes busier such that unreserved slots are harder to find, the protocol of VDL Mode 4 allows a station (e.g., aircraft) or ground station to use a slot previously reserved by another distant station. The result is that the coverage area of a station reduces in range gracefully as the channel becomes busy and there is no sudden reduction in the ability to communicate. A further advantage is that slot selection is carried out by all stations and there is no reliance on a ground station to carry out channel resource management, although in some circumstances, a ground station may be used to allocate slots.
The slot selection process consists of three steps. First, an application wishing to send data or to place a reservation to send data in the future first specifies a range of candidate slots from which a slot will be chosen. Second, the station then derives a list of available slots. The available slots are a subset of the candidate slots and consist of slots that are either unreserved or which, although previously reserved by another station, can be made available for use. Before a slot is finally selected, it is important to derive a number of available slots, typically 4, in order to reduce the possibility of more than one station selecting the same slot. For example, if there was only one unreserved slot among the candidate slots, there would be a high chance of more than one station choosing that slot, causing garbling of that slot. Third, a slot is selected randomly from the available slots.
The protocol of VDL Mode 4 allows for the use of previously reserved slots by employing the two guiding principles known as Robin Hood and Co-Channel Interference (“CCI”) protection. The Robin Hood principle allows a station operating on a busy channel to use slots previously reserved for broadcast transmission by another station as long as slots reserved by the most distant stations are chosen in preference to those of nearer stations. This results in a graceful reduction in the broadcast range of a station on busy channels. The CCI protection principle generalizes the Robin Hood principle to allow slots previously reserved for point-to-point communication between two stations to be used by another station. CCI protection is based on relative aircraft distance and assumes that even though stations may be in radio range of each other, each station can successfully discriminate the desired (stronger) signals over the undesired (weaker) ones. VDL Mode 4 defines a measure of the CCI on the basis of free space attenuation of signals with distance.
Although the STDMA protocol is appropriate for situations in which there are a large number of aircraft, it is inadequate for formation flying due to the extremely high latency rates and the complexity of the protocol.